Abstract: A locking device intended to be mounted on a fuel pipe nipple (15), which is provided with bayonet locking pins, so as to make it impossible to connect a fuel hose coupling to an outboard motor on a boat. The locking device comprises an elongated external sleeve (1), which at one end is provided with an internally situated lock (6) and at the other end with bayonet locking grooves (20,21,22,23) corresponding to the bayonet locking pins (16). The lock (6) has an axially movable locking piston (9) cooperating with an internal sleeve (14) which is axially displaceable inside the external sleeve (1) between a retracted first position, which permits relative interconnection and release, respectively, of the pipe nipple (15) and the locking device, and an advanced, second position in which the bayonet locking pins (16) are kept securely fixed in the bayonet locking grooves (22,23).